1. Field of the Invention
The present invention relates to a method of manufacturing a photovoltaic device. More specifically, the present invention relates to a method of manufacturing a photovoltaic device wherein a plurality of photoelectric converting regions are formed on a substrate and the individual photoelectric converting regions are connected in series.
2. Description of the Prior Art
FIG. 1 is a sectional view showing one example of a conventional photovoltaic device constituting the background of the present invention. Such a photovoltaic device is disclosed in, for example, U.S. Pat. No. 4,281,208 issued on July 28, 1981. The conventional photovoltaic device shown in FIG. 1 will be briefly described to the extent required for an understanding of this invention.
On a glass substrate 1 a plurality of (three in the illustrated case) photoelectric converting regions are formed comprising transparent conductive film portions 2a, 2b, and 2c. On each transparent conductive film portions 2a, 2b, and 2c there are formed overlying semiconductor film portions 3a, 3b, and 3c consisting of amorphous silicon, and respective back-side film electrodes 4a, 4b, and 4c extending to the adjacent transparent conductive film portions respectively. The semiconductor film portions 3a, 3b, and 3c each contain at least one PIN junction parallel to their surface. When the incident light impinges upon the semiconductor film portions 3a, 3b, and 3c through the glass substrate 1 and the transparent conductive film portions 2a, 2b, and 2c, photovoltaic power is generated at each PIN junction. The photovoltaic power generated by the semiconductor film portions 3a, 3b, and 3c is outputted in series fashion since the back-side film electrodes 4a, 4b, and 4c are connected with the respective adjacent transparent conductive film portions.
Various approaches have been taken for improving the energy efficiency of the conventional photovoltaic device as shown in FIG. 1. In order to improve the energy efficiency of such a device, it is essential to improve its light utilization factor. Thus the proportion of the total area of the semiconductor film portions 3a, 3b, 3c . . . actually contributing to electricity generation to the overall area of the photovoltaic device exposed to light has to be increased as much as possible. In such a photovoltaic device, however, there exist among the semiconductor film portions 3a, 3b, and 3c regions without semiconductor film, which are indicated by the reference symbol NON in FIG. 1, and this makes the total area of semiconductor film relatively small compared with the overall area of the device exposed to light. Hence, in order to improve the light utilization factor of the device, it is essential to minimize the distances between the individual photoelectric converting regions. The extent to which such distances can be reduced depends on the working precision for the individual film portions. Generally, therefore, a photo-etching technique which is known for its super-high working precision is employed.
In the case where such photo-etching technique is used, the working procedure is as follows, referring to FIG. 1. A transparent film is formed over the entire surface of the glass substrate 1, etching is then performed with a photoresist film formed for masking the areas corresponding to the transparent conductive film portions 2a, 2b, and 2c. After etching, the photo-resist film is removed, and thus the transparent conductive film portions 2a, 2b, and 2c are formed equally spaced from one another. For formation of the semiconductor film portions 3a, 3b, and 3c, also, the respective steps of forming the semiconductor film and the photo-resist film for masking, etching, and removal of the photo-resist film are required. Such photo-etching technique is indeed superior in the working precision attainable, but is likely to cause defects in the semiconductor film due to e.g. pinholes possibly occurring in the photo-resist film for masking, i.e. for defining the etching pattern or peeling of the same along its edges.
Another method of forming the respective film portions without the use of photo-resist is disclosed in, for example, U.S. Pat. No. 4,292,092, issued on Sept. 29, 1981, which features the use of a laser beam. As is disclosed therein, the laser beam is used for cutting the film in the necessary pattern; however, care is required lest the laser beam should damage other film underneath, if any.
In the above U.S. Pat. No. 4,292,092, therefore, it is proposed to select proper laser output power or pulse frequency for each film to be cut. As the experiments made by the present inventors show however, it is extremely difficult to select the laser output power or pulse frequency such that the laser beam is effective only for a particular film. When, for instance, the back-side electrodes 4a, 4b, 4c . . . are formed of aluminum, it is necessary to remove excessive aluminum by melting by laser beam, and when the laser output power is raised to a level high enough for melting aluminum, the amorphous semiconductor film underneath or in the vicinity thereof is bound to be heated to a very high temperature sufficient to be damaged thereby. This is inevitable even if the pulse frequency is selected properly. Damage of the amorphous semiconductor film is of grave consequence for a photovoltaic device. The thickness of the amorphous semiconductor film in photovoltaic devices of this type is normally less than 1 .mu.m, and considering that the amorphous semiconductor film is only up to several .mu.m thick even if a tandem junction is formed as disclosed in U.S. Pat. No. 4,292,092, the manufacturing method disclosed in this U.S. patent is not necessarily optimum.